1. Field of the Invention
The present invention relates generally to controllers for machines which operate on alternating current and particularly to current fault detection systems for AC machine controllers.
2. Description of Prior Art
Machines which are powered by alternating current (AC) are used extensively in areas such as, industry, home appliances, recreation, etc. The most common of such AC machines is a motor. Many AC motors are able to use multiple phase AC power.
Most multiphase motors are equipped with controllers which alter certain motor operating parameters, such as motor speed or torque, to fit a particular motor to the demands of the load to which it is being applied. These controllers are most often electronic controlling devices which allow more or less electrical power to the motor using known power limiting methods. These electronic controlling devices are very susceptible to current overloads, and therefore severe damage, as the motor demands high levels of power to drive an extremely demanding load.
Two methods are presently used to prevent damage to electronic motor controllers during a current overload condition. One method senses the position of the assembly which is being driven by the motor, such as an actuator arm or jackshaft, and relates that to the time taken to move to the new position. The ratio of change in position to time infers whether a current overload is occurring. If the ratio becomes too small it signals that a current overload is occurring and the motor controller is shut down by the protection system.
The motor or actuator arm positioning method is inexpensive and easily adapted to most positioning applications, however, it also has shortcomings. The protection of this type of overcurrent protection is limited to the time of mechanical feedback from the actuator arm or jackshaft, and electronic overload damage often occurs before the feedback time has elapsed. Also, an overload situation may occur which is not related to the mechanical position of the load, such as, a short circuit in the wiring or a short in the winding of the motor. Due to these limitations, the mechanical feedback overload system does not provide adequate protection to electronic motor controllers.
The second method presently used to protect a motor controller from over current damage is to provide direct current measurement of each phase of the motor being controlled and to shut down the entire motor control system if any of the phase currents go above a preset safety level as provided in FIG. 1. The signals from each current sensing device are input through a logical OR gate to a digital controller. If a digital high signal is sent to the digital controller from the OR gate, the digital controller sends a system shut down signal to the motor and motor controller.
In this overcurrent protection system each current sensor communicates with the digital controller. However, the digital controller must be protected from the surges of power which commonly occur in the motor control system. The presence of these power surges demand that there be ground isolation between the each current sensing device and the digital controller. Ground isolation is usually accomplished through known opto-isolators, as shown in FIG. 1, which are expensive. Also when any of the phase currents of the motor goes above the threshold safety level the entire system is shut down until reset by an operator. Often this entire motor system down time is unnecessary because only one phase of the motor is in an overcurrent state, and a 3-phase motor can usually continue running on two phase power.
Thus it is seen that a current overload protection system for 3-phase motors is needed which does not shut down the entire motor control system in an single phase, overcurrent condition yet provides adequate protection to the motor control system in an inexpensive manner.